TypeError：无效文件：使用xlsxWriter导出数据帧时无

Question

我一直在我的Python / Flask应用上使用XLSXWriter将数据帧导出到Excel。直到今天，我设法在一个工作表上导出我的数据帧;现在我试图在一个文件中的不同工作表上导出多个数据帧。

当我在本地尝试时（请参阅附带的第一个代码），一切都很完美。但是当我写入IOStream时，我收到以下错误：

TypeError: invalid file: None

以下是功能本地代码：

    path_user = expanduser("~").split("\\")[-1]

    xlsx_path = r"C:\Users\{}\Desktop\ExportAllGraphs2.xlsx".format(path_user)
    writer = pd.ExcelWriter(xlsx_path, engine='xlsxwriter')

    count = 0

    for df in df_list:

        sheet_title = df["title"].split(' ', 1)[0]    
        sheet_name = "{}_{}".format(count, sheet_title)

        df_header.to_excel(writer,startrow = 0, merge_cells = False, sheet_name = sheet_name)
        df["df"].to_excel(writer,startrow = len(df_header) + 5, merge_cells = False, sheet_name = sheet_name)

        workbook = writer.book
        worksheet = writer.sheets[sheet_name]
        format = workbook.add_format()
        format.set_bg_color('#eeeeee')
        worksheet.set_column(0,50,20)
        worksheet.write(len(df_header) + 4, 1, df["title"])

        count += 1

    writer.close()
    os.system('start excel.exe "{}"'.format(xlsx_path))

这是无效的实时代码：

    output = BytesIO()
    writer = pd.ExcelWriter(output, engine='xlsxwriter')

    count = 0

    for df in df_list:

        sheet_title = df["title"].split(' ', 1)[0]    
        sheet_name = "{}_{}".format(count, sheet_title)

        df_header.to_excel(writer,startrow = 0, merge_cells = False, sheet_name = sheet_name)
        df["df"].to_excel(writer,startrow = len(df_header) + 5, merge_cells = False, sheet_name = sheet_name)

        workbook = writer.book
        worksheet = writer.sheets[sheet_name]
        format = workbook.add_format()
        format.set_bg_color('#eeeeee')
        worksheet.set_column(0,50,20)
        worksheet.write(len(df_header) + 4, 1, df["title"])

        count += 1

    writer.close()
    output.seek(0)

    return send_file(output, 
    attachment_filename="test_export_{}.xlsx".format(current_date), 
    as_attachment=True)

我错过了什么吗？